Personally, I found that 99% of renamers suck (the ones I tried at least).
So instead I use a dumb vbscript that fills an array with the names of all the files, then loops thru it renaming all files. There's a "blank" spot in that loop, where you can type the code to manipulate the file names yourself (actually, there's pre-written ones commented out -- for the lazy). It's trivial to write, and it works great if you can handle a very simple scripting language.
Replacing underscores with spaces would be as simple as:
CODE
s = Replace(s, "_", " ")
(you wouldn't even have to write that, just un-comment a line and put the underscore/space in there -- google for vbscript string replace or if you need explanations)
Sure, it's not the most user-friendly "app", but I have yet to find an app that's half as powerful (again, assuming you can write a simple script), doesn't have confusing options and things you have to lookup (e.g. use %D for date and then use these complex parameters for formatting it a certain way), etc. You could even use regular expressions if you wanted to (more complicated though).
Not for everyone I guess, but it's still the best thing I've "found" by far...